JP H6-101534 (hereinbelow termed Patent Document 1) describes an evaporated fuel processing device. In this Patent Document 1, a sensor configured to detect a fluid density of air introduced to a canister and a sensor configured to detect a fluid density of a purge gas sent to an engine from the canister are provided, and a concentration of the purge gas is calculated based on a ratio or a difference of the fluid densities thereof. Further, a flow rate of the purge gas to be introduced to an intake passage is determined based on the calculated gas concentration, and the flow rate of the purge gas sent to the engine is adjusted by using a purge valve that is controlled on a duty basis.